It is natural and normal for the semen to flow back out. Very very little goes up into your cervix (mainly the sperm, not the semen).

It is also not all semen that flows back out. A fair amount of your own juices are also mixed in. The semen also become watery once ejaculated into your vagina to allow the sperm to escape from it.

You can put a pillow under your hips for about 10 minutes to keep the semen inside. After that you can wipe all you want. Doing kegal squeezes over the toilet can help you to clean out this excess flow-back fluid from your vagina and keep you drier over the next day.

There is also some thought that you having an orgasm just after he ejaculated can cause more of the sperm to get sucked up by the cervix and research have shown that an orgasm can transport the sperm right up to the ovaries in less than 5 minutes. The research showed that this effect is most pronounced from orgasming from 45 seconds before he ejaculate to about 2 minutes after he ejaculated.